1. Field of the Invention
The invention relates to, in an auxiliary drive wheel-side differential unit for a four-wheel drive vehicle that includes a power transmission member configured to transmit driving force from a driving source to auxiliary drive wheels in a four-wheel drive mode and that disconnects the power transmission member from the auxiliary drive wheels in a two-wheel drive mode, a technique for preventing a fully differential state of a differential gear unit provided in the auxiliary drive wheel-side differential unit when the power transmission member is disconnected from the auxiliary drive wheels.
2. Description of Related Art
There is known a four-wheel drive vehicle in which a two-wheel drive mode or a four-wheel drive mode is selectively carried out. In the two-wheel drive mode, driving force is transmitted from a driving source to right and left main drive wheels. In the four-wheel drive mode, driving force is transmitted from the driving source to the right and left main drive wheels and the right and left auxiliary drive wheels. In the four-wheel drive vehicle, there is known an auxiliary drive wheel-side differential unit for a four-wheel drive vehicle configured to disconnect a power transmission member from the auxiliary drive wheels in the two-wheel drive mode. The power transmission member transmits driving force to the auxiliary drive wheels in the four-wheel drive mode. This is, for example, an auxiliary drive wheel-side differential unit for a four-wheel drive vehicle, described in International Application Publication No. 2010/019641.
In the auxiliary drive wheel-side differential unit for a four-wheel drive vehicle, described in International Application Publication No. 2010/019641, a control coupling (multi-disc clutch) is provided in an axle between a differential gear unit and one of right and left auxiliary drive wheels. In a two-wheel drive mode, the differential gear unit is disconnected from the one of the right and left auxiliary drive wheels by the control coupling, with the result that a power transmission member, such as a propeller shaft, that transmits driving force from a driving source to the auxiliary drive wheels in a four-wheel drive mode is disconnected from the auxiliary drive wheels.